Thursday, 25 March 2010

A new member of the human evolutionary family has been proposed for the first time based on an ancient genetic sequence, not fossil bones. Even more surprising, this novel and still mysterious hominid, if confirmed, would have lived near Stone Age Neandertals and Homo sapiens.

"It was a shock to find DNA from a new type of ancestor that has not been on our radar screens," says geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. These enigmatic hominids left Africa in a previously unsuspected migration around 1 million years ago, a team led by Pääbo and Max Planck graduate student Johannes Krause reports in a paper published online March 24 in Nature.

The researchers base their claim on DNA from a finger bone belonging to a hominid that lived in the Altai Mountains of central Asia between about 48,000 and 30,000 years ago.

Anthropologists have generally assumed that hominids left Africa in a few discrete waves, starting with Homo erectus about 1.9 million years ago. Neandertal ancestors left between 500,000 and 300,000 years ago, followed by humans around 50,000 years ago.

But the new genetic sequence supports a scenario in which many African hominid lineages trekked to Asia and Europe in the wake of H. erectus, Pääbo suggests.

This curious sequence was extracted from a piece of finger bone unearthed in 2008 at Denisova Cave in southern Siberia's Altai Mountains. Previous excavations of stone and bone artifacts in the cave indicated that modern humans and Neandertals lived there periodically beginning at least 125,000 years ago. Few fossils have turned up at the site.

While retrieving DNA from presumed Neandertal fossils in November 2009, Krause noticed an unusual mitochondrial sequence. Mitochondrial DNA is located outside the cell nucleus and inherited from the mother.

Krause conducted tests to confirm that the newly recovered mitochondrial DNA came from an ancient hominid rather than from bacteria or researchers who had handled the fossil. Using advanced techniques for fishing DNA fragments out of fossils, the team then assembled a complete mitochondrial genome for the Denisova individual. The same approach has yielded ancient DNA sequences for Neandertals (SN: 3/14/09, p. 5) and a Greenland man who lived 4,000 years ago (SN: 3/13/10, p. 5).

The researchers compared Denisova mitochondrial DNA to complete mitochondrial sequences from 54 people who are living today as well as a human who lived in Siberia about 30,000 years ago, six Neandertals from more than 40,000 years ago, a modern pygmy chimpanzee and a modern common chimp.

Mitochondrial DNA from the Denisova fossil differs from that of humans at almost twice as many chemical positions as Neandertal mitochondrial DNA does, Krause says.

"That number of differences is good evidence for a new hominid because simple variation can't account for it," remarks geneticist Morten Rasmussen of the University of Copenhagen.

Assuming that mitochondrial DNA ancestors of humans and chimps diverged 6 million years ago, the researchers calculate that a mitochondrial ancestor common to the Denisova hominid, Neandertals and modern humans lived between 779,300 and 1,313,500 years ago.

A common mitochondrial DNA ancestor of modern humans and Neandertals lived more recently, an estimated 321,200 to 618,000 years ago.

Krause and Pääbo are now directing an effort to extract nuclear DNA from the Denisova fossil. Comparisons of Denisova, Neandertal and modern human nuclear DNA are needed to confirm that the finger bone comes from a new hominid species and to check for signs of interbreeding with Neandertals or humans.

For now, the researchers refer to the Denisova hominid as "X woman," although its sex remains undetermined until nuclear DNA can be examined.

X woman's finger bone came from a soil layer that has yielded bracelets and other artifacts usually attributed to humans, Krause notes.

"What we can say for now is that there were at least three different forms of hominids living in the Altai Mountains around 40,000 years ago," Pääbo says. At that same time, Homo floresiensis, better known as hobbits, occupied the Indonesian island of Flores (SN: 5/10/08, p. 7). Hobbit DNA has yet to be recovered.

In a comment published with the new report, geneticist Terence Brown of the University of Manchester says that further ancient DNA studies will "possibly increase the crowd of ancestors that early modern humans met when they travelled into Eurasia."

Anthropologist Ian Tattersall of the American Museum of Natural History in New York City agrees. Hominid evolution over the past 6 million to 7 million years includes at least two dozen species, in Tattersall's view. It was "practically routine" for two or more species to live in the same general area at the same time, he says.

Tattersall regards the new mitochondrial DNA sequence as so distinctive that, unless disproved by further evidence, it must represent a new type of hominid.

In contrast, anthropologist Erik Trinkaus of Washington University in St. Louis views the new genetic data skeptically. "I don't know what to make of this, at least not until there is more substantial fossil material than a partial finger bone," he says. "It may be going too far to propose a new hominid."

Trinkaus, who sees fewer species in the hominid family than Tattersall, cautions that biologists have difficulty identifying different species even among living primates. For example, groups of baboons that usually live apart as apparently separate species sometimes aggregate and interbreed, muddying their classification.

Pääbo acknowledges the complexity of finding new hominids in mitochondrial DNA, which in animals such as mice can pass from one species to another via interbreeding. "But there are massive genetic differences between X woman and both Neandertals and modern humans," he says.

A survey of 200 sites in 20 countries around the world has found that bisphenol A, a synthetic compound that mimics estrogen and is linked to developmental disorders, is ubiquitous in Earth's oceans.

Bisphenol A, or BPA, is found mostly in shatter-proof plastics and epoxy resins. Most people have trace amounts in their bodies, likely absorbed from food containers. Its hormone-mimicking properties make it a potent endocrine system disruptor.

In recent years, scientists have moved from studying BPA's damaging effects in laboratory animals to linking it to heart disease, sterility and altered childhood development in humans. Many questions still remain about dosage effects and the full nature of those links, but in January the U.S. Food and Drug Administration announced that "recent studies provide reason for some concern about the potential effects of BPA on the brain, behavior, and prostate gland of fetuses, infants and children."

The oceanic BPA survey, presented March 23 at an American Chemical Society meeting in San Francisco, was conducted by Nihon University chemists Katsuhiko Saido and Hideto Sato. At an ACS meeting last year, they described how soft plastic in seawater doesn't just float or sink intact, but can break down rapidly, releasing toxins. In their new findings, they showed that BPA-containing hard plastics can break down too, and found BPA in ocean water and sand at concentrations ranging from .01 to .50 parts per million.

As for what those numbers mean for public and environmental health, it's hard to say. BPA can cause reproductive disorders in shellfish and crustaceans, and doses below a single part per trillion can have cell-level effects, but the path from water and sand to ocean animals needs to be studied.

One disturbing possibility is that BPA could bioaccumulate, with animals eating BPA-tainted animals that have eaten BPA-tainted animals, finally reaching high concentrations in top-level ocean predators and the humans who eat them. For that to happen, BPA would have to be stored in fatty tissue, rather than passing quickly through the body.

"That's a really difficult, unsettled question," said Shanna Swan, a University of Rochester environmental medicine specialist who wasn't involved in the survey.

In a 2009 Environmental Health Perspectivesstudy of BPA concentrations in people who had recently fasted, Swan found that BPA levels remained high longer than expected. It's possible that BPA indeed accumulated in their fat, said Swan. They could also have picked up BPA from as-yet-unappreciated non-dietary sources, such as household dust or leaching from PVC water pipes. Or both scenarios may be true.

The BPA contamination found by Saido and Sato likely comes from a mix of boat paint and plastic. About three million tons of BPA-containing plastics are produced each year. The United Nations estimates that the average square mile of ocean contains 46,000 pieces of plastic trash.

"Marine debris plastic in the ocean will certainly constitute a new global ocean contamination for long into the future," wrote Saido and Sato in their presentation.